As an example of optical films such as phase difference films for use in display devices such as liquid crystal display devices, there is known a film manufactured by stretching isotropic raw material films to give orientation to molecules.
It is desirable that such optical films do not cause much color tone change of display devices even when the observation angle is shifted, and that such optical films can uniformly provide effects, such as phase difference compensation effect, over a wide wavelength range.
As examples of the known phase difference film that can uniformly provide the effects such as phase difference compensation effect over a wide wavelength range, there are known films obtained by stretching a composition containing a plurality of resins (Japanese Patent Application Laid-Open Nos. 2001-194527 and 2001-42121). However, with these films, color tone changes due to observation angle shift cannot be reduced.
For reducing color tone changes due to observation angle shift, it is desired that retardation Re at an incident angle of 0° (in the direction normal to the film) is brought as close as possible to retardation R40 at an incident angle of 40° (an inclination angle of 40° with respect to the direction normal to the film). In order to obtain such an optical film, it is desired that, e.g., the refractive indices nx and ny in the directions of in-plane principal axes and the refractive index nz in the thickness direction satisfy the relationship of nx>nz>ny.
However, most resins have positive intrinsic birefringence values, stretching of which results in smaller nz than nx and ny. Resins having negative intrinsic birefringence values such as polystyrene are unsuitable for optical films in terms of inappropriate hardness and flexibility of the stretched film made thereof. Therefore, it is difficult to obtain a useful stretched film that satisfies the relationship of nx>nz>ny. As methods for solving this problem, there have known a method in which a plurality of species of films are laminated, and a method in which a stretched film is shrunk (Japanese Patent Application Laid-Open Nos. Hei 03-24502, Hei 03-141303, and Hei 05-157911). However, these methods are accompanied by difficulties in controlling the direction of lamination and shrinkage, which renders the manufacturing process complicated, and renders manufacture of a film having a large area difficult.